船舶轮机英语Lesson12课件

Lesson 12 第第12课课MARINE MEDIUM SPEED DIESEL ENGINE The term medium speed diesel engines is currently taken to mean an engine with an operating rotational speed of between 300 to 1200 r/min.This group of engines is now entirely of the trunk piston type,mainly operating on the four-stroke cycle and the majority is now fitted with exhaust gas driven turbochargers and charge coolers.*Rationalization(合合理理配配置置)of cylinder layout(布布置置)and numbers of cylinders has been carried out in the power range of 300 to 750 kW by most manufacturers,resulting in the basic configuration(结结构构,外外形形)of either six or eight cylinders in line,or alternatively(或或者者)six or eight cylinders in a Vee form.For power requirements greater than,say,900kW,8,12 and 16 cylinders Vee or twin bank engines are used.Cylinder and crankcaseThe cylinders of medium speed engines are water cooled.The bore of each cylinder is formed in a liner which can be replaced when worn out.The cooling water is in direct contact with the outer surface of the liner as a result of which it is termed a wet liner.A number of cylinder liners are enclosed in one cast iron casing to form a cylinder block.The cooling water jacket is common to all the cylinders but there are often dividing walls to ensure that each cylinder receives the right amount and flow of water.Doors are provided on the cylinders casing,through which the water spaces may be cleaned and inspected when overhauling the engine.In an alternative construction each cylinder is enclosed by a separate water jacket.On a bedplate is mounted a casing termed the column(机机架架)which forms the crankcase and supports the cylinder block.In many designs the force produced by the reactionof the cylinder heads to the gas pressure in the cylinders is transmitted from the top of the cylinder block directly to the crankshaft main bearing by through bolts(long-tie bolt贯穿螺栓贯穿螺栓).Smaller engines can be made more rigid than large ones and may not have through bolts.In addition,because the casing are not so big and heavy,the column and cylinder block may be made in one piece.For Vee-form engines the top of the column is shaped to give two sloping surfaces on which the cylinder blocks are placed at the correct relative angle to each other.Doors are formed in the column of the both in line and Vee engines to give maintenance access to the crankshaft main and connecting rod large end bearings.CrankshaftsCrankshafts of the medium speed engines are almost invariably solid forged from a single piece of steel.The cranks of a multi-throw(多多拐拐的的)shaft are set at angles to each other giving a firing order for the engine.Although the crankshafts of medium diesel engines appear to be robust(坚坚固固的的),they rely on the support of the main bearings to develop their full strength.When a crankshaft has to be handle outside the engine,it should be carefully supported in a manner which will avoid imposing(施施 加加)high bending moments on it.In the engine it is essential to ensure the bearings carrying it are in good alignment(对中对中).Connecting rodsWith a few exceptions medium speed diesel engines have trunk pistons with the result that piston and connection rods have to be fitted together before being assembled into the cylinder.For one or two types of engine,the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the crankcase door.However,this design tends to result in a high engine and therefore the piston and rod is more usually withdrawn upwards passing through the bore of the cylinder.In case of engines have separate water jackets,the whole cylinder assembly with the piston and rod inside may be removed as a single unit;The aperture(孔孔)in the frame for the water jacket being much larger than the cylinder bore gives much more room for the connecting rod large end to pass.Connecting rod large ends are either of fixed centre(固固定定中中心心式式)or marine type(船船用用式式,中中心心可可 调调 式式)design.In the latter design,the large end bearing is separate from the rod.A distance piece,with ground faces,known as a compression plate may be interposed between the rod and the bearing housing,its thickness being chosen to ensure the correct compression ratio(压缩比压缩比).Cylinder linersCylinder liners are made from close-grained cast iron(细细晶晶粒粒铸铸铁铁).*They are simple cylindrical shapes flanged at the top end to provide location and a means of securing them in the cylinder block or to the water jacket.Immediately below this flange there is often a joint ring which may be of copper(铜铜)or in some designs of a heat resisting rubber(耐热橡胶耐热橡胶).The lower end is fitted with rubber rings to form a seal for the bottom of the water space.As well as stopping water leaks into the crankcase these rubber rings may be arranged also to prevent oil from the crankcase entering the water jacket.The upper part of the liner bore where the top piston ring reaches the top of its travel suffers greatest wear.This is because at this point the ring comes to rest and reverses its direction of motion and it is difficult to maintain an adequate film of oil between the surface of the ring and liner.Also the gas pressure is highest when the ring is in this position,forces it hard against the liner and,of course,the top of liner is hot from the repeated combustion cycles which tends to dry off(弄弄干干)any oil there is.Additional,tiny particles of carbonaceous matter are formed by the combustion processes,some of them may be abrasive and over a period of time an accumulation builds up in the groove around the ring leading to wear promoting(促促进进)condition.Obviously,wear will be reduced if the top part of the liner is kept reasonably cool.Liners are sometimes specially shaped to fit the jacket in a way that promotes cooling without sacrificing(牺牺牲牲)strength.The bores of cylinder liners when new have a specially prepared surface designed to aid the running in of the piston rings and the liner.It is slightly rough in order to retain the oil and to promote rapid wear in.Some designs of liners have a deposit of chromium plate on their bores.The chromium plate is of a porous(多多孔孔的的)nature to provide an oil retaining surface.Reading materialINSPECTION OF MAIN PARTSPiston,rings and cylinder linersThe piston should be inspected to see that it is free from cracks and that there are no ridges(隆隆起起)or grooves on bearing surfaces other than the ring grooves.If scores(划划痕痕)cover more than 5 per cent of the contact area with the liner the piston is unacceptable.The axial clearance of each piston ring should be measured at about four place round its circumference(圆圆周周),this is done by pressing the ring down on the bottom land of the groove.This axial clearance should be between 0.00045 and 0.0015 of the piston diameter.The rings should be inspected to see that they are free from ridges,cracks and grooves.The gap clearance of the ring should be measured by inserting each ring in turn in a ring gauge or by placing it in the bottom unworn portion of the cylinder liner and using a depth gauge or a piston crown to make sure that it is positioned symmetrically(对称地对称地).The gap is measured by feeler gauges between the ends of the rings.The top compression ring gap should be between 0.006 and 0.009 of the cylinder bore,the other rings between 0.004 and 0.006.If the gap clearance of a piston ring is too large the thickness of the ring should be checked and the ring replaced if it is apparent that it is badly worn.The new ring should also be checked for gap clearance and filed(锉锉,锉锉平平)on the butts(端端面面)to the correct minimum dimension.If the axial clearance(轴轴向向间间隙隙，天天地地间间隙隙)of the piston rings is unacceptable,the ring should be measured to see whether it is the ring or the grooves that is worn.If it is the grooves,and rings of oversize axial thickness are available,the grooves can be re-machined to suit such a new ring.The bore of the cylinder liners should be inspected and measured.They should be free from scores,cracks and ridges.If the liner is chromium plated then patches(斑斑点点)where the chromium has worn off should be carefully examined to see if gas blow past is occurring.*A cylinder dial gauge(千千分分表表，量量缸缸表表)should be used to measure the wear of liner by taking readings parallel to and at right angles to the center line of the crankshaft at the top of the top piston ring travel,half way down the liner bore,and at the bottom of the liner bore where it is probably not worn.Any liner which is badly cracked or scored causing gas blow past should be replaced.The chromium plate on the bore of a liner is beginning to wear through,the liner should be replaced.The wear of a cylinder liner should not exceed 0.004 times the bore diameter.If it is a chromium plated bore liner the permissible wear will be depend on the thickness of the chromium plate.A cylinder liner will have to be replaced if it is cracked or if there is excessive wear or scoring of the bore,or pitting and corrosion on the outside,a ridge will have formed in the liner bore at the top of the piston ring travel,this should be removed by stoning if the top ring is renewed.Crankshaft and bearingsOnly one main bearing half shell should be removed at a time,the other being left in place to give the crankshaft adequate support.Each journal and pin should be examined for ridging and grooving.Ridging are formed in the way of oil grooves in the bearing shells,if they do not exceed 0.0015 inches in height they are acceptable unless a new bearing shell is to be fitted.Grooves are formed by abrasive particles entering the bearing and scoring the shaft,if the effective bearing area has been reduced by more than 10 percent remedial action should be taken.The diameter of the crankpins should be measures using a micrometer(千千分分表表).Measurement should be taken at two positions along the length of each pin at eight or twelve positions round the pin(i.e.at 45 or 30 intervals).The difference between the maximum and minimum diameters of a crank pin should not exceed of the maximum permissible bearing clearance.It is usually not possible to use a micrometer in order to measure the diameter of the main journal with the crank in position in the engine.However,some measurements can be taken by a special bridge gauge(桥桥规规)and feelers(塞塞尺尺)as shown in Fig.5.The measurements are always taken in the horizontal plane and the crankshaft turned through 30or 45 intervals for each successive(连续的连续的)measurement.Bearing shells should be rejected(报报废废)if:a)The bearing metal is cracked or broken up;b)The bearing surface is grooved over more than 10 percent of its area;c)The arc(弧弧)of contact between the journal or pin and its bearing is less than 140and the area of contact is less than 90 percent within the arc;d)The steel back has been fretting(磨损磨损)in the housing;e)There is less than 90 percent contact area between the back of the bearing and the housing.The nip(压压 隙隙)of each bearing should be measured.With the shell and cap assembled as for running the nuts should be tightened the full amount and a check made that the clearance between cap and housing is zero.The nuts should then be slackened back to finger tightness and the clearance between the cap and the housing measured at each side.The sum of the reading is the nip.For thin shells it should usually be between 0.0008 and 0.0018 of the journal diameter.After ensuring that the nip is correct the bearing clearance should be checked.The bearing cap and its associated half shell are removed leaving the other half in position.Two pieces of pure lead wire are placed across the journal at position 1/3 r and 2/3 r along its length.The cap and shell are replaced and tightened up until the nip is zero.The cap and shell are then removed and the thickness of the lead wire measured by a micrometer.It is important to use only pure lead wire which is very soft and to choose a thickness of wire which is greater than the normal allowable clearance but not greater than twice this amount.The clearances should be within the manufacturers limits.As a guide these will be approximately 0.1mm to 0.2mm for white metal bearings and approximately 0.25mm to 0.4mm for copper lead or tin-aluminium bearings with crankshaft journal diameters in the range from about 200mm to 400mm.